Quite a bit, considering I purchased a CM gold run and a Garrosh kill because my guild will NEVER get those done with the current chaos. Those were around 70k together. And my DK is getting to work on Warrior Tier 3, at minimum shoulders, helm, maybe boots.
But mostly time, almost everything I do is for it, be it farming a set from lower tiers or getting a mount / pet to match it (my rogue thanks you, Anzu).
What's the point of money if you sit on it, what's the point of gear when it will be replaced in a patch or two, a Tmog set can be eternal.

With my priest:
- full Plagueheart
- full Faith
- most parts of Frostfire

With my monk:
- full Dreamwalker
- full Bonescythe

With my Shaman alt (which I don't even play )
- full Cryptstalker

A lot of crafted BoEs which I didn't need, but wanted for transmog.
A lot of greens that I couldn't be bothered to farm myself.

- Teebus Blazing Longsword

The list goes on.
If I had to guess, I'd say more than 600k.
Don't really care about it. I keep ~1 mil. in case a nice mount etc. shows up on BMAH, but other than that, I don't need the gold for anything else anyway.

A lot. I'm a role player and there are a couple of characters who change regularly. I've spent a combination of gold, justice, valor, honor and time on collecting sets. One character even had to level leatherworking to get hers. Counting that plus the other things I've bought for various characters, as well as the cost of transmog itself (especially on higher level characters), I would guess it's up around the 400k mark and one of the things contributing to keeping me poor, on some realms.

Less than 1000g total for all 11 characters that I would consider ones I actually play. I thrift store shop for transmog. I think of a look and a color scheme, go to the AH, search for pieces with the cheapest listed first and just find a way to make the cheap stuff work together. If its got a price tag of over 100g total, I think of a different look\color.

I haven't made any single big purchases. Just a lot of little gear that adds up (no clue...guessing around 4k). I know I have spent a lot on transmogging itself. I feel neurotic that way, trying to find the "perfect" look for a toon, thinking I've found it, and then going through the process again in a few days. I'm never satisfied

The one big purchase that is most tempting, haven't pulled the trigger, is the Sword of Wizardy or whatever it's called. I think there are similar models with that lightsaber look.

A few thousand here and there. It's mostly a time investment for me because I farm items on three characters (hunter main, dk alt, warlock alt) until I get everything. I have bought Cryptstalker boots and gloves thought off of the BMAH for super cheap though (under 10k).